RMS Focal Spot: The REAL Focal Spot?

There is a well recognized and urgent need for an objective method to establish a meaningful label describing focal spot performance, or "effective size." The present standard is based on deriving a "size" descriptor from the measurement of the distance between certain features in a radiograph made with one of the various focal spot cameras. Such features are "preceptible edges", "extent of the intensity distribution", or "blur zones". However, (1) the interpretation and/or location of such features is often subjective, (2) the derived number is in many cases not a measure of an "effective" size indicative of focal spot performance or resulting geometrical unsharpness of the image. An observer independent size descriptor is the "RMS Focal Spot", which also appears to describe focal spot performance well. A slit camera and a densitometer are the basic tools for its derivation. We are proposing the principles of both a standard labeling method and a "minimum effort" quality control procedure for focal spots.